1. Field of the Invention
The present invention relates to a developing apparatus which is installed in a color image forming apparatus using an electrophotographic process or the like and which is used to visualize an electrostatic latent image formed on an image bearing member.
2. Related Background Art
In a full-color image forming apparatus, for example, a copying machine, there are usually utilized a method which comprises using four photosensitive drums, developing an electrostatic latent image formed on each photosensitive drum with cyan, magenta, yellow and black toners, conveying a transfer material to the photosensitive drum by a transfer belt (a belt-shaped transfer member), and then transferring each color toner image obtained by the development to the transfer material to form the full-color image thereon; and another method which comprises winding a transfer material around a surface of a transfer drum (a transfer material holding member) by an electrostatic attractive force or a mechanical function such as a gripper, conveying the transfer material to one photosensitive drum opposed to the transfer drum, transferring a toner image formed on the photosensitive drum to the transfer material, and then repeating the steps of the formation and the transfer of the toner image for four colors, thereby obtaining the full-color image.
In recent years, the development of a technique is increasingly demanded where in the image forming apparatus, a small-size paper such as a cardboard, a card and a postal card is applicable as the full-color transfer material, in addition to an ordinary paper and a film for an overhead projector (OHP).
For the image forming apparatus in which the transfer belt is used to convey the transfer material, since the transfer material is conveyed in a plane state, images can be formed on various transfer materials and an application range is extensive. However, a plurality of toner images need to be correctly superposed in a predetermined position of the transfer material, and even a slight difference in registration deteriorates an image quality. To enhance a registration precision, a conveying mechanism of the transfer material is complicated, and the number of components is disadvantageously increased.
On the other hand, for the image forming apparatus in which the transfer material is wound around the surface of the transfer drum by adsorption, and the like, and is conveyed, when a cardboard large in basic weight is used, because of a rigidity of the transfer material, a trailing end of the transfer material causes a failure in a tight fit with the surface of the transfer drum, and as a result, a defective image caused by the transfer is easily produced. Even in the small-size paper, the defective image may be produced by the same cause.
Therefore, various image forming methods using intermediate transfer members have been proposed. For example, a full-color image forming apparatus using an intermediate transfer drum, that is, a drum-shaped intermediate transfer member is described in U.S. Pat. No. 5,187,526. However, in the description of the patent, shapes and constitutions of toner particles are not concretely mentioned. In Japanese Patent Application Laid-Open No.59-125739 described is a method in which a toner image formed with a toner with an average particle diameter of 10 .mu.m or less is transferred to an intermediate transfer member, and the toner image on the intermediate transfer member is transferred to a transfer material. As one toner manufacturing method, a direct manufacturing method by a suspending polymerizing method is described. For a transfer in an image forming process of the publication, a pressure transfer or a sticky transfer is performed, which has a problem that the surface of the intermediate transfer member is easily contaminated by the transfer of a multiplicity of materials.
In the image forming apparatus which uses the intermediate transfer member, after the toner image is once transferred to the intermediate transfer member from an electrostatic latent image bearing member such as the photosensitive drum, the image is again transferred onto the transfer material from the intermediate transfer member. Therefore, a toner transfer efficiency needs to be enhanced more than before.
To solve the problem, the present applicant has proposed an image forming apparatus which uses an intermediate transfer member as shown in FIG. 4 (Japanese Patent Application Laid-Open No. 7-209552).
In FIG. 4, numeral 101 denotes a photosensitive drum as a first image bearing member, the photosensitive drum 101 is rotatively driven in an arrow direction in the drawing, and in the process of rotation the surface of the photosensitive drum 101 is uniformly and negatively charged by charging means 102. Subsequently, exposure scanning is performed by exposure means 103 ON/OFF controlled in accordance with first image information, and a first-color electrostatic latent image is formed on the surface of the photosensitive drum 101. The latent image is developed by a first-color negatively charged developer included by a first developing apparatus 104a, and visualized as a first-color toner image. Here, toner for use is manufactured by the suspending polymerizing method, contains a low softening point material, and has a shape coefficient SF1 of 100 to 150.
The first-color toner image visualized as described above is electrostatically transferred on the surface of an intermediate transfer member 105 rotatively driven as a second image bearing member, in a position opposite to the intermediate transfer member 105 (a primary transfer). The process is reiterated a plurality of times, and second-color, third-color, and fourth-color latent images serially formed on the surface of the photosensitive drum 101 are respectively developed with developers different in color by second, third, and fourth developing apparatuses 104b, 104c, 104d including the respective developers. By transferring the obtained toner images onto the intermediate transfer member 105, a color image in which the four-color toner images are superposed on top of each other is formed on the intermediate transfer member 105.
The color images on the intermediate transfer member 105 are electrostatically transferred in unison onto a transfer material 107 which is conveyed to a nip of a transfer roller 106 rotating in contact with the intermediate transfer member 105 (a secondary transfer). The transfer material 107 with the color images transferred thereon is conveyed to a fixing device 111, where the color images are heated and fixed to the transfer material 107.
Primary transfer residual toner remaining on the surface of the photosensitive drum 101 during the primary transfer, and secondary transfer residual toner remaining on the surface of the intermediate transfer member 105 during the secondary transfer are removed from the surfaces of the photosensitive drum 101 and the intermediate transfer member 105 by cleaning means 109, 110, respectively.
In the above-described proposed image forming apparatus, by using as the toner a substantially spherical toner (a polymerized toner) manufactured by the polymerizing method and having the shape coefficient SF1 of 100 to 150, as compared with non-fixed shape toner (a crushed toner) manufactured by a conventional crushing method, the transfer efficiency can remarkably be enhanced. Even if the use of the intermediate transfer member results in two transfer processes, a color image sufficiently excellent in respect of the transfer can be reproduced.
Moreover, the polymerized toner contains a low softening point material such as paraffin wax, and the like. Without applying a large amount of silicone oil, or the like to the fixing roller of the fixing device 111, the occurrence of offset can be prevented. Also in this respect an excellent full-color image can be obtained.
A developing method of the developing apparatus 104 (104a to 104d) is not particularly limited. Generally examples of the developing method for use in the color image forming apparatus include a non-magnetic monocomponent method and a non-magnetic two-component method. In the latter non-magnetic two-component method, since a two-component developer with toner and carrier mixed therein is used, with consumption of the toner the mixture ratio needs to be adjusted, which raises a problem that the apparatus constitution is complicated and enlarged.
Therefore, in recent years, the former non-magnetic monocomponent method has been frequently used, and as a developing apparatus suitable for the developing method, a constitution as shown in FIG. 5 is prevalently used.
In FIG. 5, numeral 115 denotes a developer container, 112 denotes a developing sleeve as a developer bearing member provided in an opening portion of the developer container 115, the developing sleeve 112 is rotatively driven in an arrow direction in the drawing. A developing blade 113 and a toner supplying and collecting roller 114 abut against the surface of the sleeve.
The developing blade 113 as a toner regulating member is constituted by bonding an elastic member 113b of urethane rubber, and the like on the side of the surface of a support member 113a formed of phosphor bronze or another elastic material opposite to the developing sleeve 112, elastically abuts against the surface of the developing sleeve 112, and has functions of forming a thin layer of toner on the surface of the developing sleeve 112 and applying a triboelectric charge to the toner.
The toner supplying and collecting roller 114 is constituted by coating an outer peripheral surface of a core metal 114a formed of SUS, and the like with an elastic member 114b of urethane foam, and the like, and has functions of supplying non-magnetic toner contained in the developer container 115 to the surface of the developing sleeve 112 and of scraping off from the surface of the developing sleeve 112 a toner returning to the developer container 115 without contributing to development in a developing section opposed to the photosensitive drum 101.
The developing apparatus constituted as described above is excellent for the non-magnetic toner of the conventional crushing method, and can preferably form the thin layer of the non-magnetic toner sufficiently provided with the triboelectric charge on the surface of the developing sleeve 112.
On the other hand, when the above-described developing apparatus is applied to the above-described polymerized toner which is a toner formed by the polymerizing method, having the shape coefficient SF1 of 100 to 150, having a substantially spherical shape, and containing the low-softening point material, the following disadvantages are caused.
Generally a powder smaller in particle diameter than the toner is externally applied to the toner for the main purpose of enhancing and stabilizing the triboelectric charge. Even in the polymerized toner, for example, a fine powder of silica, or the like is added as an external application agent, and the external application agent adheres to surfaces of individual toner particles in a covering manner.
However, since the polymerized toner is spherical, the adhering force of the external application agent to the toner tends to be weaker as compared with the crushed toner. The external application agent present on the surface of the toner on the developing sleeve 112 is gradually liberated from the toner as the developing sleeve 112 continues to rotate. Therefore, when the toner not having contributed to the development is removed by the toner supplying and collecting roller 114, the toner can be removed, but the liberated external application agent is insufficiently removed, and remains on the surface of the developing sleeve 112.
If the adhesion of the external application agent to the surface of the developing sleeve 112 continuously occurs, a film of the external application agent is shortly formed on the surface of the developing sleeve 112. Although in the nip portion between the developing sleeve 112 and the developing blade 113 which abuts against the sleeve 112 an electric charge should originally be applied to the toner, this is prevented by the film of the external application agent, and a sufficient charge cannot be applied to the toner. Furthermore, the insufficiently charged toner slips out of the nip portion, and phenomenon so-called dripping of the toner occurs.
In Japanese Patent Application Laid-Open No. 8-185041, it is proposed that on the surface of the developing sleeve 112 a coating layer containing a main component of resin, and carbon, graphite and another conductive fine powder or a solid lubricant dispersed in the component be formed to prevent the external application agent from adhering to the surface of the developing sleeve 112.
Additionally, to attain a high image quality of a recent image forming apparatus of 600 dpi, 1200 dpi, or the like, it is intended to obtain small particle diameters or fine particles of the toner. In order to faithfully reproduce the latent image on the photosensitive drum and obtain a high resolution, a fine toner whose weight-average particle diameter is in the range of about 4 to 7 .mu.m needs to be used. Moreover, the developing apparatus is constantly requested to have a reduced running cost, a high quality and a high reliability, and also have a high durability and an extended life.
If the fine-particle toner is used, and in the developing apparatus, the developing operation is repeated particularly under a low-humid environment, it becomes difficult to obtain a sufficient image density.
Specifically, the charge amount of the toner with which the developing sleeve 112 is coated becomes excessively high by contact with the developing sleeve, and the toner has difficulty in moving to the latent image on the photosensitive drum 101 by a reflection force with the developing sleeve surface. Moreover, since a high charge amount of toner is present in a lower layer of the toner layer on the surface of the developing sleeve 112, the toner present in a top layer thereof cannot have an opportunity to contact the developing sleeve surface, and it becomes difficult to obtain the electric charge. As a result, since either upper layer toner or lower layer toner on the developing sleeve cannot easily move to the latent image, the image density is lowered, and the upper layer toner is further easily scattered.
The phenomenon tends to be promoted because the particles become finer and a surface area per unit weight of the toner is accordingly enlarged.
As a countermeasure, a method of appropriately roughening a surface roughness of the developing sleeve 112 is considered. Thereby, a conveying property of the toner by the developing sleeve is enhanced, while the rolling or switching of the toner under the pressure contact of the developing blade 113 on the developing sleeve surface is promoted. As a result, on the developing sleeve a toner layer uniformly having an appropriate electric charge amount can be formed.
The roughening of the surface roughness of the developing sleeve 112 can be realized by increasing an addition ratio of carbon or graphite in the resin forming the coating layer, but first the method has a problem in respect of durability because the coating layer becomes brittle and is easily worn. Particularly in the non-magnetic monocomponent developing apparatus, since the developing blade 113 and the toner supplying and collecting roller 114 originally pressure-contact the surface of the developing sleeve 112, wear resistance of the developing sleeve 112 needs to be enhanced.
Secondly, it is difficult to control the surface roughness of the developing sleeve 112 by the coating layer to which the fine particles of non-uniform shapes are added, and the shape of the developing sleeve surface unfavorably becomes non-uniform.
As shown in Japanese Patent Application Laid-Open No. 3-200986, it is proposed that a conductive coating layer, in which in addition to a solid lubricant or carbon and other conductive fine particles, spherical particles are dispersed in resin, be formed on the surface of the developing sleeve 112.
The proposed method has merits that the shape of the surface of the developing sleeve 112 is uniformed, the conveying property and triboelectric charging property of the toner are uniformed and that the wear resistance of the developing sleeve surface can be enhanced. However, even when this method is used, it is difficult to stably and excellently form the toner layer of the fine-particle polymerized toner on the developing sleeve.
Therefore, when the fine-particle polymerized toner is applied to the developing apparatus, influencing factors having influence on a developing property, and the like, such as the particle diameter of the polymerized toner, the surface roughness of the developing sleeve, the particle diameter of the spherical particle to be added to the coating layer of the developing sleeve surface, and the like need to be considered, and it has been desired that these factors be appropriately defined.